For some rss readers they have issues with the images having "mtime" at the end of the url. Is there a way to output an image without this?
?mtime= should only appear for Assets images that are being served from a source that has Expires settings set (so, either an S3 or Google Cloud source).
This is done done for cache busting purposes. If you upload a new file, it's mtime value will change causing a different URL and, in turn, forcing the user's browser to serve the new image, instead of the cached version.
So to serve images without that you would have to turn off Expires setting for that source.