Rephrase some negative-sounding messages around the site
Since I registered I have felt there were several things that come across negative. And I'm sure you don't want them to. Example: when I go to sign in sometimes the word Register is over the place I should login and I can't get to it. So I am forced to go to "Join Now" and it has a place that says if you have an account click here to login. The. The first page is see is ACCESS DENIED. YOU DO NOT HAVE ACCESS TO THIS PAGE" A similar message occurs when you can only download certain files before you do the monthly fee. To me this is a put off & should be phrased better. I have many years of marketing experience so these kinds of things shout at me :).
Also, in terms of the files themselves I know there is the option to get transparency but not on all files. I use photoshop, elements 15, Indesign and other photo editing software platforms. I have run into problems with some of the elements in particular. Like a ribbon. If it doesn't have transparency, I go the route of select all pixels then select pixels to remove the background. Most of the time that hasn't worked here so I used the quick selection option. I have spent a lot of time trying to clean those up. It just seems to me it is a simple thing for a true designer to do. And save the customer time and frustration. I use several blogs and/or websites and transparent PNG is being used throughout.
Reaching your limit for the day. I would suggest saying: We are sorry but according to our guidelines you have reached your maximum of 5 kits today. Then maybe give the option to upgrade right beside it. Again, the way it pops up now makes the user feel like less of a patron.
This is just a personal thing as I am not interested in the more clip art images of animals, cute little things, etc but I'm sure you have a large customer base that does use these. I was thinking if you have a testimonial area where you highlight some awesome work that your customers have sent to you. I think that would engage others to look around more. Or maybe a section of more professional pieces. Just something to think about. Or maybe not.
I have found lots of papers I can use and I see several designers that have excellent work. When I start a project I am looking for crisp, clean and vibrant colors. These make for s much more eye catching piece.

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Susannah Pani commented
I also get a negative feeling when you get that ACCESS DENIED message - usually when I, due to time differences, attempt to download a blog train kit too early. It does not jive with the generally positive and friendly wording on the rest of the site.