In case that you are searching for a way to recover images from hidden .slm files generated by Gallery Lock app for Android, your best bet would be to try some of the picture recovering apps, but it is unlikely a dedicated slm to jpg converter exists.
A .slm file is basically a .jpeg, it has just first 1024 bytes of data additional data put in the front. If you are experienced enough, you could delete them yourself using a hex editor and "recover" your JPEG, but an image recovering software would do the work better. This is pretty much the only thing you need to know how to recover files from safe gallery lock.
Of course, if you could always unhide and (thus recover) the images in the Gallery Lock app, provided you know the PIN.Open SLM file Open JPG file