Is there a way to send only an Image with a link and some alt text for each item in an RSS feed?
I looked at the enclosure
tag but this is only for videos
You should use the enclosure tag within item to include the image. You can use it for images by setting the correct Mime Type (for example: image/jpeg) and including the image size as the "length" attribute. The length attribute doesn't need to be completely accurate but it's required for the RSS to be considered valid.
Here's a helpful article that discusses this and other options.