Here is a layered psd for recolours of the Base Game "Transcendency" painting with a passepartout. Optional photo effects (vignette; sepia tone) are already built in, as are the shadows inside the frame and inside the passepartout, and the paper colour. You may need to add some noise to the vignette if you have pictures with large monochrome areas (to avoid posterising).
This is Photoshop CS3, I don't know whether it's backwards compatible.
Result when you just slap a greyscale picture onto the "picture goes here" layer will look like this in-game:
The area inside the passepartout is 214x170px. You may want to nudge the frame shadow down by 1 or 2px, right now it's a little illogic (but I noticed that too late – the light will mostly come from above, after all).
Tip: create the recolour with the separate "Overlay" layer turned off, and only turn it back on right before exporting – this will give you a more realistic preview (at least on my monitor it does, that's why I made two overlays). The paintings look incredibly bright in-game and need to be toned down by a LOT. Note that this template is for the unchanged, original TXMT -- if you edit that, you can probably make the picture less bright that way too.
Important: You need to resize the final .png to 256x256 and mirror it horizontally before importing it to SimPE.