Sometimes .tmp temporary files are pictures or fragments of pictures and these can be just renamed. So the first thing you can try is to change suffix from tmp to jpg and see how the file looks when opened in grapics editor / viewer.
In most cases however , temporary files are just chunks and fragments saved for temporary reasons. You can find some recovery utilities that advertise this function (picture recovery from temp) but whenever it will actually work for your .tmp files is not certain. In any case, direct tmp to jpeg conversion is highly unlikely.Open TMP file Open JPEG file