I remember that extension but I didn't user it because it didn't cleanup after it self (old not needed images stayed in cache).

what is "your phone"? you mean an app? I know about text caching (I don't know if frehsrss has an option to get original page for RSSs that has just a simple text that redirects to full page), but even inoreader that had that (if i remember correctly) didn't have image caching.