Welcome to the modding FAQ. (work in progress) Here we have a list of questions that are common to get while writing a mod. And the fix for the problem (if someone managed to find a fix) feel free to edit this page and add more Q and A's.
Questions / Fix
Make sure your path is right : __mod-name__/map-name/2nd-map-name/sprite.png
Make sure you have valid locale mappings. Create a "locale" directory with an "en" subdirectory and create a "item-name.cfg" file. It should contains something like: [item-name] itemx=Item X itemy=Item Y Do the same for your entities, but put them in an "entity-name" section/file.
Make sure your entity contains the right amount of lines for that type of entity, for example a chest needs 1 picture for the entity to work but a wall needs 20. Look in the factorio base/prototypes/entity/entity.lua. And you can see that different entities need different amounts of pictures.