Some ? most? all? microprocessor based devices like STB, DVP Layer etc. have a checksum routine inside. This routine will be active at startup and should prevent to install unvalidated (i.e. hacked) code on the device. And if this checksum routine is not too generous, it will also calculate the checksum across the picture data itself (reason: picture data needs a lot of space, and is a fine place to store some code there, as you dont need always some background pictures or are able to disable the view of a background picture)
So first you have to identify if there is really a JPEG format of picture within your STB. Probably the picture is stored in JPEG, but it might also be possible that the picture is stored as plain bitmap.
After this, you might find a way to modify your software (including to generate a checksum)
Then you have to transfer the modified file in your STB. I would assume that there is some available transfer software from the box vendor to upgrade the firmware.