It actually seems the problem is solved now that I have sucesfully converted png files to binary RBG565 swap files with the online converter.
The files are still stored in the filesystem (I didn’t conver in a C array), but I don’t need the decoder for these files, I think this is the reason it doesn’t crash anymore.
The tricky part is that just using the files as they are give a distorced image, this is because the first 4 bytes are for the header, as it is explained in this discussion: Possible issues when using indexed binary images - General discussion - LVGL Forum
@kdschlosser Thank you very much for your support, maybe I will still need help to squeeze more memory later, but for now it seems to be ok 