Here's my perspective as a user of TP and noimg...
As I understand it, the noimg stats tell you how many clicks are being made by visitors who do not load images on your site. The assumption being that if a visitor doesn't load images then they are not a real person - so, the smaller the figure the better.
I have this feature enabled on my sites and I typically see a noimg% of between 5 and 10 for most 'good' trades, a few are 10 to 15, and notrades, bookmarks, noref etc are up in the 20 to 30 range (eg, search engine bots don't not typically load images).
I believe that this feature does use cookies though so the noimg traffic will include a component of nocookie too.
If you are seeing 50% across the board then it's possible you have a server or setup issue - BUT, if you are seeing 50% for a bunch of trades but 10 to 20 or so for others then I'd say it's likely that the 50% is a real figure and you should investigate those trades.
For example, have a careful look at your server stats and see whether the number of REAL images being loaded by visitors is consistent with the number of visitors that are loading your home page. Does the typical visitor actually load ALL the images on your page - for example, possibly the img.php element is too far down the page and not being loaded soon enough?
hth