Thanks to Doctor Dew for compositing and restoring the poster image above!
The M.U.S.C.L.E. poster was a mail-away offer from Mattel. One could be ordered by sending two four pack proof of purchases or one 28-pack proof of purchase. At first, the poster featured all-pink figures. As the line expanded into non-pink colors, the poster was updated with the new colors. Shown here is the all-pink poster.
If you're looking for large, detailed scans of the figures, check out the MUSCLE Archive!
Above, left to right: 4-pack offer, Close-up of 28-pack poster offer
The 28-pack Poster
So what figures exactly are shown on the 28-pack poster? Alas, we may never know. While it appears that there are definite figures on the 28-pack poster, when it's scanned at high resolution, you don't see much additional detail. Rather, you see more of that dot-printing technique than anything. This picture of the poster seems to be best viewed at a distance, but if you're curious, here's a high resolution scan.