Hello Max,
Yes, PIX code (at least up until the current versiont 6.3.4) is identical across all models, so you can use the same image for 501, 506, 506E, 515, 515E, 525 & 535.
PIX 7.0 (released at the end of this month) will be supported on 515, 515E, 525, 535, but not on 501 and 515(E).
Hope that helps.
Sincerely,
Binh