In this case you actually don't need to conjugate the verb pintar. There are two alternatives: one using the infinitive, another using the gerund.
- Quiero ver a mis amigos pintar.
- Quiero ver a mis amigos pintando.
The difference is very slight, mostly the same as in English ("I want to see my friends paint" vs. "I want to see my friends painting").
You'd only need to conjugate the verb if you were using a full subordinate phrase, for example:
- Quiero ver a mis amigos mientras pintan. ("I want to see my friends while they paint")
- Quiero ver a mis amigos que están pintando. ("I want to see my friends who are painting.")
In all of the above, note:
- Quiero... is more natural than Yo quiero...; you can safely drop the pronoun.
- You must say «Ver a mis amigos», not *«Ver mis amigos». This is the so-called "personal a".
P. S. Useful link (advanced, in Spanish): this answer of mine about infinitives in subordinates with verbs of perception.