The best way how you can convert JPEG picture (.jpg, .jpeg) to a 3D model for 3D printing (STL format), would be to se a program capable of generating 3D meshes from pictures. This is more or less what jpeg to stl conversion represents and is typically done for the purpose of 3D printing.
It is likely that you can find plenty of tutorials all over internet for this procedure, even some informative "how to" YouTube videos do nicely explain how to convert pictures for 3D printing. In any case, you will need a software that basically can convert 2D image to 3D model.Open JPEG file Open STL file
A 3D CAD translation software
A 3D general-purpose modeling program
3D modeling application for Mac