If the designer gives you all the assets (images, redline document etc) then it’s ok otherwise you have to collect the design assets from photoshop or sketch. That’s why you have to learn minimal photoshop and sketch.
From my experience, I have learned it’s not a good practice to disturb designer any silly issue like (need a transparent image, image optimization etc).
I am not a designer but I know the minimal use of photoshop and sketch and trying to learn/use new tools and plugins of the sketch in my free time. The sketch is very popular now. Most of the designer currently using the sketch. You can go with the sketch.
Do practice, Do study from other designers design, gather knowledge and create a Behance account. Upload your project in Behance and you’ll get positive/negative/improvement/ motivation/inspiration feedback from the Behance user. My Behance account.