Do you have illustrator? If you do, you can place your guitar images in Ai and use image trace (on high fidelity photos) to create vectors of the guitars, paint / color them as you wish and paste them back into Ps as smart objects. Scaling the guitars as vector images will maintain the quality if you choose to increase the size. The high fidelity trace will not be exactly the same as the photographic image but since you plan on painting them anyway you may like this style for your poster. If you don't have illustrator then you should work with the guitar images at 100% or in other words at the same size you want them to be when printed so that you aren't enlarging them. This image is 27" wide now at 72 dpi... do you have a higher resolution file or are you pulling these down from the web? Generally speaking you can reduce image size without losing quality but not the other way around. Again this image looks good at 27" and if the width of your poster is 24" it should hold up.
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