When creating a Custom Image set for your ESP Forms Based authentication, it's possible Internet Explorer will not display your images and will look something similar to the below image. This is due to "Mime Sniffing" which is used to examine the contents of a file to determine the file format and carries out comparisons.
Remove "X-Content-Type-Options" HTTP header.
1. Rules & Checking > Content Rules > Create Rule
Rule Type = Delete Header
Header Value = X-Content-Type-Options
2. Navigate to Virtual Service > Advanced Options > HTTP Header Modifications > Response Headers and apply new rule. If the Virtual Service contains Sub VS, you will apply the rule on the Top level VS.