Try using a hair dryer on HIGH setting to soften the adhesive holding the decal on. If you have a real heat gun use that instead but you need to be careful to avoid cooking the decal and maybe the pain underneath. Solvents aren't likely to help owing to the decal film (these days likely as not PSA vinyl) 'cause the adhesive's underneath.

Once the decal itself is off, you can usually use denatured alcohol (ethanol with a tad of something else in it to make it toxic and un-drinkable so not taxable as spirits) to dissolve the left over adhesive, or a citrus based paint remover. Whatever you use try it on a small, inconspicuous area of the painted frame so you know ahead of time whether what you're intending to use will damage the paint or not.

If simply 'first-surface' (means on top of) PSA vinyl the hair dryer ought to work for you.