The problem with being on the web is that all sorts of people what to use stuff for free. That is why it is good to limit size and ad watermarks! You are under no obligation to help them out, unless you want them to really use your images for free. A while back I offered up a standard email you could use to reply to those that are not offering a business proposition upfront. It seems a good way to save your time and energy while still being polite.
Thank you for your interest in my artwork. I create my art as a business, and make my living from it. If you have a particular project you would like me to give you a price quote on, send me information on the use or product, the size of image reproduction, the number of impressions to be made, and the length of time/ number of times you will be printing or showing this project.
Again, thank you for your interest,
This is usually enough to separate out the serious from those looking for free use.
If it is a school student asking for report images (and you are inclined to respond at all), send them an 600 to 800 px JPG at 50% quality, and ask for attribution and web URL link, or stick it right in the image. You can also request it not be posted on line (only works for honest users). An 600-800 px image at 50% quality should be fine for any personal use that is not being presented at full page size, and minimizes the chances that a *significant* commercial use could be made from it.
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