In many ways you are trying to create a Powerpoint looped playout using CasparCG playout.
I agree with didikunz that the official client is not good at automation as it does not support looping a group. He is also right that you will find SuperConductor is better. I have no experience of PlyCast.
Later in this post I will describe how to acheive what you have asked about using the standard client as it may be useful for future readers of this topic to be able to create a simple slide show that may, for example, be used to entertain an audience before a programme recording starts.
Using SuperConductor
This client supports looping of groups. My experience is that you need two channels of Caspar enabled - a real one such as your channel 4, and a virtual one that is used to prepare the still as a finite duration object. In SuperConductor create a new group. Drag a still from the library to the second channel. Use the edit tools to change the duration from 0 (infinite) to 10.0 seconds. Drag the now finite duration still into the end of the real channel timeline. Repeat as many times as needed for the sequence. Look for the group loop control at the left side of the group display and enable looping. Start the sequence playing.
You can edit the sequence whilst it is playing, adding new clips or adding transitions between the stills.
Using a Video Edit package
If your still sequence does not change too frequently you can use the free-to-use DaVinci Resolve editor to create a movie that the standard client can loop. There is a simple trick so that minimal work is needed to create a new clip.
Create a folder that contains the 20 images of the sequence. Use any recognised image encoding such as JPEG, PNG, BMP. Name the clips Image_01, Image_02, Image_03 etc. Make a second folder that is used as the media source in the editor. Copy the contents of folder 1 into this media folder.
Open DaVinci and create a timeline for the project. Import the stills from the media folder you named for your sequence. Drag the clips to the timeline, setting durations and adding transitions as desired. If you want all changes to have transitions you place a half-duration image 1 at the start of the timeline, and add the second half of that image duration at the end of the timeline. Add the transition at the boundaries between the images but not to the start and end of timeline.
Switch to the render tab, set the video codec. I recommend an intra frame codec such as DNxHd (or ProRes LT if you are working on a Mac edit station). Add the clip to the render queue then tell DaVinci to render the queue of clips to your chosen target folder and file name. Transfer the clip to the CasparCG server, add to the playback channel setting loop active. Play the clip.
When you want a new set of pictures create the set in a new folder, naming them Image _01 etc. Copy these files to the folder DaVinci is using as the media sources for the still sequece playout. Open DaVinci and select your stills image project which reads the new set of stills into the timeline because we used the same media element names. All you need to do is hit the Render Queue button and confirm you want to re-render. Copy the new file to CasparCG.
Using SVT Standard Client
This description assumes the stills are in the media library and the client has done a refresh to see the latest media list. Drag the stills from the library into a rundown. This can be done in a stand-alone rundown tab (recommended!!). Add image transitions such as mix if these are required.
Select all the stills and convert them into a group. On still number 2 set the Output Delay to 10000 (10 s), set still number 3 delay to 20000 etc. When you select the group item and hit the F2 play button image 1 plays immediately. After 10000ms the client sends a play command to show image 2 etc. In the screen grab below I used 5 seconds rather than 10 seconds. The delay offsets are shown in each still image control widget.
Now for the trick that makes things loop. Use the client configuration to add a named OSC output such as self-trigger. Set the target IP address to 127.0.0.1 and the port to 3250 (remote OSC control for client 2.2 and later). Make sure the OSC Input Control is enabled (box just above the OSC Output destinations setup zone).
Enable remote control of the stills group in the Inspector, giving the group a simple UID name such as group1. Add an OSC output to the end of the image group. In the OSC output properties select the self-trigger destination. Set the path to /control/group2/play , set the message to type integer and the message value to 1. Set the delay on the OSC command to 200000 (20 * 10 seconds expressed in ms).
Create a second OSC output and make this the sole member of a new group. Set the OSC output to send to the self-trigger address, setting the path to /control/group1/play, again sending an integer message with a value of 1. Enable remote control on the second group giving a UID of group2.
Select the first group with the images. Press F2 to start playing the items in the group, each starting at the time offset from the start of the group. After 200 seconds the OSC item sends a play message to group2. In turn this sends a play message to group1 and the looping process is operating.
