Pictures are awesome. Profiles are awesome. And profiles are extra awesome with pictures. That's the consensus, as least.
But, I think most of you agree that pictures that are beyond the width of the DSi screen are pretty unpleasant to scroll past. So it is highly recommended that you scale them down to a manageable size.
Here's how to do it while on PC:
1) Find a website that allows you to resize a photo. I suggest the Photobucket subsidiary http://tinypic.com, as it does not require registration.
2) Retrieve the image you want. If it's on your computer, just upload it to the website. If it's on the internet, sometimes you'll be able to paste the image url. If not, save the image and then upload it to the website.
3) Resize the picture so that it is less than 240 pixels wide, which is the second dimension in a given set like 160 x 120; in that example, 120 is the width. The best preset is 320 x 240. So as long as it's at or less than 240 pixels, it's usually okay.
4) Submitting the image will bring up a sample image and a few versions of embedding. Just copy the raw image url (starts with "http://" ).
5) Paste it in your blog, profile, or group between the image tags:
Just so you know, Photobucket is also an option. It does require sign-up, but offers more customization in the way of editing your pics. Me, I use Photobicket.