two quick tricks for metal.
1. When cleaning use fantasitc cleaner. it is available in the super markets. it leaves no residue and stips things nicely. that is why they use it on stove tops.

2. Mask off the area you want to have your picture mounted on with masking tape. Use 400 grit sand paper and lightly sand the surface. it will give you the tooth they talked about in the other forum. this also with the masking keeps the areas you have of the aluminum not covered by the picture from showing the scratch marks of the sand paper. Afterwards clean using the fantasic. Once you have mounted the picture if there are any small areas of the sandpaper scratches left on the surface near the pciture, take 3000 grit sand paper and lightly sand that area. You cn thin take a bit of car polich and use a qtip to get the surface shiny again. If it is a very small area get a popsicle stick and glue the 3000 grit paper on one side of it. you can then use it more acurately in very thin swipes. Use a tooth pick instead of the q-tipo to polish with. If you have a dremel or flexible shaft, you will have the tips to do this more effectively.

Good luck