I sometimes receive jpg photos in emails that are too small..when I try to enlarge ..photo is...?

Unfortunately to save on data size the photos are typically 72 resolution, so when you go to enlarge it the photo will be fuzzy because there aren't that many pixels to work with. It's a trade off, you can have a huge photo (dimension-wise) at low resolution to begin with, or make a smaller photo with higher resolution. if possible, see if you can get a copy of the higher resolution photo that way when you are resizing it there will be more information in the file and it won't be blurry.

People send smaller file sizes to fit into the email providers limitation. Photos can be many megabytes, one photo can consume more than the entire amout of space you are allowed to send in an email. The way around this is to shrink the file size by compressing and making the photo smaller. But you lose image data when doing that. Once that data is gone it cannot be recovered so you can never get the image back to the the original size. Photographers do this for a different reason. They dont want people to abuse their copyrights by stealing their pictures. They shrink the file size so the viewer can see the image at a smaller size on a computer screen, but cannot make it any bigger for their own personal use. When the someone intends to steal the image and enlarge it for printing, or even using it as a screen background, the quality of the shot quicky evaporates and it becomes pretty much unusable. Bottom line, once the image data is gone by shrinking the file, it cant be recoverd for enlarging.

A JPG photograph is composed of pixels, and that's all the information that's there in the picture. A big, detailed photograph has many pixels; a small photograph has only a few. If you zoom it up, you'll get bigger pixels, but there isn't any information to provide any more detail. You're used to regular photos, where the pixels are incredibly small and even in small photos there's often more detail to be seen by blowing the picture up. With a digital picture, what you see is all you get. You don't actually see big, blocky pixels when you expand a JPG because the format is really more clever than that. It's capable of "interpolating" between two pixels. That is, if there is a mostly-red pixel right next to a slightly-less-red pixel, it will put a new pixel between the two that's somewhere between the two. That makes the enlargement smooth rather than blocky. But it can't create new details. A small pink patch will become a big pink patch, but there isn't any information to say, "Oh, this is a face, we should add eyebrows and such." If there were eyebrows, they've become nothing more than a darker smear on the face, slightly darkening some pink pixels. I've rather oversimplified here. The JPG format is astonishingly clever, and it's capable of cramming a huge amount of information into a very small file, so that you can see more detail than in any other format. But it's not magical, and it can't make details materialize out of nowhere. All you can do is ask for them to send a higher resolution picture.