As it has been expainedexplained here, beside of multiple extensions that perform ad or script blocking you may aware that this may happen by file names as below:
Particularly in the AdBlock Plus the character string "-300x600" is causing the Failed to Load Resource ERR_BLOCKED_BY_CLIENT problem.
As shown in the picture, some of the images were blocked because of the '-300x600' pattern in their name, that particular text pattern matches an expression list pattern in the AdBlock Plus.