Remove 360px PCP image restriction, replace with CSS resizing

Problem: the PCP image attachment is arbitrarily limited to 360px. Users do not understand how to resize their own images prior to upload.

Solution: remove the 360px PHP restriction, replace it with a more flexible CSS max-width image resizing strategy. All modern browsers can support this.

As discussed with Dave Greenberg. Sponsored by Baykeeper and CiviTeacher.