your bottom row of images are in gold leafed type frames (the left looks more like copper though). gold leaf is just very thin sheets of gold (like, thinner than a leaf) that is rubbed onto a surface which has been pre-treated with a sort of glue called size. gold leaf can be applied as a decorative detail on the relief sections of a pattern, or all over a whole surface. its like a variation on gold spraypaint or other gold toned metallic finishes, visually. think 'old masters' for gold leaf frames and you'll get an idea.
i reckon a simple, not ornate, burnished gold leaf frame might be nice on the top image, but only if the paint used in the face is more gold than yellow. if its yellow, it'll just look odd. i'd not do gold leaf for the sunflower, cos it'd not go with the yellow or with the style of work.
if you wanna display them together, i'd probably window mount them on a white or very slightly off-white mounting board, and go with a simple black frame. can't really go wrong with that- its got impact, it'll draw your eye to the image, not from it, and black tends to go with anything.
i'd definately not go with an ornate or elaborate frame for the sunflower image, cos its pretty uncomplicated and modern. while that works for the other pieces, cos they're complex and use the same tones, it'd maybe be odd for the more dramatic brightly coloured pieces.
have you asked the artist for suggestions? if he's got the skills to put together the work, he'd most likely have an excellent eye for what kinda frames would work with them.