If you look through the source code for JPearman’s Animated GIF demo, there is no prefix or directory name for the gif file - try passing just “pic.bin” and see if it works.
Then, from the source code for lv_img_set_src() function it looks like it expects the image file with the specific header data. (there are several places where it is called in PROS).
I assume you already have such header…
If there is still no luck, gifdemo/include/gifclass.h has vex::Gif::Gif() and vex::Gif::render() methods that load gif image and display it on screen using direct calls into vexOS.
I understand that you want to do it with PROS and LVGL, but it has an examples of calling vexDisplayCopyRect(), which is also used by PROS.