Okay I have a real cheap black glossy ACOUSTIC guitar.
I want to HANDPAINT my guitar and also attach some paper printed photos and so on, to make it look a personal special one. I am willing to paint with acryllic colors (I am not planning to paint professionally), but I just want the paint and the other decorations to stay clear when I apply the wax on it. (I am planning to use the "Gerlitz GNO No.1 Carnauba Guitar Wax" which I got recommended from someone.)

1. Should I remove the existing finish? I know that it is better to have a thinner face but I don't know how it will go. Maybe it is better at least to make it rough enough for the paint to stick on?

2. I might paint it a bit thick because I want rough brushing patterns to be on it and the paint won't stay the smoothest since it is acrylic + If I attach some photos and stickers on it, will it be impossible to apply the wax on it? I want my guitar to look like it is containing the photo inside a glass cover. If the waxing is thick enough then will it cover the rough parts?
hopefully it's a cheap guitar - thick paint and paper are going to keep the wood from resonating. btw, the wax isn't going to give the gloss finish you're hoping for. wax - which i don't use on my guitars - doesn't create a gloss finish; it's there to protect an already glossy finish. you need to use clear coat of some kind and then probably know how to polish it right to get a glossy finish, and chances are you'd need to fill the paper with something first so it's not porous when you apply the clear coat.

i strongly suggest you find a cheap electric guitar to do this with - if you don't mind working with clear coat, which you need to be careful with as it easily sticks to hair and dust while drying.
Quote by Skeet UK
I just looked in my Oxford English Dictionary and under "Acoustic Guitar", there was your Avatar and an email address!
The only time I tried painting a guitar was with a home-built resonator guitar I made. I wanted to cover up the rather ugly "fill" I'd made for the soundhole:

So I painted it black. (No pics of the final product!)

I lightly sanded the entire surface to be painted, and then applied several coats of water-thinned artist-grade acrylic paint. I very lightly sanded these coats with very fine sandpaper, and left the last coat as is...
Then, I applied brush-on Minwax brand clear urethane finish.
The result was pretty decent.... Not super-smooth like a factory finish but not bad.

As far as applying photographs or any other "mixed media" (as we artists say...) You would want to use clear acrylic medium. This is an excellent, transparent glue and should not affect the photos in any way.

Do that BEFORE you apply any final clear coat.

How much this will affect tone is debatable...If this is a cheap guitar it's likely got a plywood top at any rate and unless you go crazy with the paint job I would not expect any great effect.

Folks used to do this all the time back in the good old hippie days....