Missing Images - determined to be ad blocking software run amok

Only some images display, link info missing

[joergnw10]

I was made aware of an image display problem on another computer. Didn't think much of it as the website worked fine on my pc and also in different browsers.
Had a look at it on this other PC today and discovered that only about 50% of images appear. The rest aren't even replaced with the red X - they just don't seem to exist - even the link info in the source is missing.
The same problem on this PC with IE and Firefox, so it's not likely to be a browser problem.
Checked the links with Xenu and they are all ok. Browsing directly to the URL's of the images also works. I know that this PC displayed things just fine until about a week ago.
The website is pure html, no php / database... Does anyone have an idea what could cause this?

[tedster]

Ad blocking software? It's a frequent cuplrit even for images that are not ads, since it often filters by pixel dimensions and character strings within the file name.

What you are describing - no red X in a placeholder, just no image - is exactly the kind of result that ad blocking creates.

[joergnw10]

Bingo! Must have been an update of Norton Internet Security that caused it. I just hope not too many people have the same settings as my website would not be worth much without the pics.
I guess I'll have to look into how to prevent them from being regarded as ads. Just looking at filenames etc I can't see why some pics are blocked and others are not. Also they are all the same format and roughly the same size. Is there some known easy prevention (ie using alt tags or similar)?

[ChrisBolton]

I would love to know exactly what causes this. Is it actual images brought in through the HTML or background images brought in through CSS, or both? It may have something to do with the file name, the alt attribute or even the title attribute if you use it for images. If you can, post the details of the ones that do work and the ones that don't. This is something everyone should be taking careful note of.

[tedster]

Here's a thread from 2004 that has a lot of detail on what Norton was blocking at that time:

Norton Blocking Legitimate Content [webmasterworld.com]

[joergnw10]

It looks like it could be a combination of image size and something else.
The pages that show this problem have lists of products, each with a thumbnail image of 120x60 (blocked according to the thread from 2004). They are not part of the CSS.
All images are produced using the same program with the same settings.
I have not been able to determine anything in the filenames that could cause half of them to be blocked.
I will alter their size and see if it helps. Will have a go after Christmas.