Well, This have been going on for years now, I know that this topic have been posted previously, but that is since 2018, now I wonder if the admins actually read that topic anymore, so here I start it again. I have previously also complained about the exact same issue. Copy icons under dude/images folder, it display on dude and randomly just disappear. Sometimes a few days, and sometimes after a month or more. When you right click icon and choose appearance, go to image tab, then it will just be on none. search for the icon on the drop-down box, its gone. Search for the file on the images folder, file just gone. And it is not even the same images, just randomly any one. Re copy the file back to the folder, then set icon again, it will work again for a day, week, month or so. I have tried different file formats, resize the icons, rename to short filename, you name it, the problem just keep on creeping back. This has been going since V6, up to today (July 2020). Looks as if dude, ROSv6 and ROSv7 have the same issues with development. Lots of features requested, that is mostly present with contending devices, and still lacking today, years and years of development resulting in only baby steps for a product that is widely used around the world.
I have also tried different products with the dude, x86, RB1100x4 Dude edition, RB-M33, CCR1036 router, all with the exact same issue as above. Including adding FTP. Nothing changes for me.