Hmm, try enlarging the entire texture of the object. For example, if the original texture (so the painting including the frame and such) of the Maxis object you're editing is 256x512 or even smaller, resize it to 200% or more (as long as one or both sides is 1024px). This way you don't have to resize your new painting so dramatically, and it will become HQ in-game. Remember to use DXT sizes though, or else it won't show up correctly (so 512x512, 512x1024 or 1024x1024). Also, when you're importing the image in SimPE, use DXT5 with Sharpen set to 'none'.