Hi @anna.boccio,
That is correct, Kaltura MediaSpace [KMS] channels are category objects [or records, depending on your perspective] with special attributes.
In regards to your question, the thumbnail per channel is kept in a custom metadata profile on each relevant category.
The profile’s name is channelThumbnail.
You can learn about how to work with custom metadata by going through this tutorial:
https://developer.kaltura.com/workflows/Enrich_and_Organize_Metadata/Working_with_metadata;step=1
This tutorial explains how to work with ENTRY custom metadata objects/records whereas you’ll need to work with CATEGORY metadata but otherwise, the flow is similar and so are the service and action names [endpoints].