I wrote several papers on such a topic. I called it end-to-display encryption. But it is more commonly called "pixel-domain encryption" There are several easy ways to do this. My way included adding a header with RSA encrypted AES Keys.
In essence, you take the picture, extract the pixels, XOR with the bitstream created through AES in Countermode, increase the counter with every iteration, convert that to pixels, save as PNG (important, not jpeg or any other lossy compression, you lose bits with that, and decryption needs every bit to work properly). Thats it. And the same way back.
But sorry, no tool available. But its easy to implement.
I may extend on the thing with the JPEG in the question. Encrypting JPEG in the pixel domain will not work properly. The compression is lossy, so you lose information. There are compression methods on JPG, but they are much more complex, because they work mostly on the DCT factors, not on the pixels itself. These can also be viewed as pictures. That said, its another application field, as i wanted to decrypt pixel streams in the HDMI cable, that would not have worked as intended, or only with much more complex systems.