Why is my file exporting pixelated?

er,

"Sadly, I noticed it when I loaded it on the website."

Is it your own website, or some platform? The reason for this is that a number of platforms are notorious for wrecking whichever quality is careful obtained.

In any case, the exported close up image just seems to show a close up, so if the pixelation is worse you may simply have an image size that is too small and spread too thinly over the screen.

If I may say so, this looks like a classic error in checking image quality. They are made of pixels so if you zoom in you will see the pixels. Never zoom over 100% when checking for pixelation, or you will always find it. 

er,

Concerning colour (brightness), it depends on your (lack of) colour management.

Concerning pixelation, this may sound unforgivably outdated and boring, sorry:

If you wish to have PNGs (PNG24 (also (little) known as PNG32, it holds 24 bit colour and 8 bit Alpha channel (transparency)), of course) look crisp and clean, it is important to have the

images in the exact desired final pixel x pixel size, forget about resolution which may actually lead to wrong sizes and hence blurriness. It is easiest and safest to work at the desired

size.

A very common unsuitable way is to Export to PNG (remember to use PNG24 and use Transparency for artwork to be in front of different backgrounds) with a medium or high resolution, such as

300PPI.

To retain the clean and crisp artwork, a PNG must be created at exactly the pixel x pixel size that it is to be used for, or at sizes that are powers of 2 times as large (2, 4, 8, and so

on).

For clean and crisp artwork avoid JPEG.

It is also important to have the artwork and also the Artboard placed fully within integer/whole pixel X and Y values in the Workspace. Otherwise the resulting image will become a bit

wider/taller and the extension(s) will be empty and therefore be white.

Therefore, the safest way is to create the artwork at the final pixel x pixel size and use a corresponding Artboard, then either use the Legacity Save for Web (where you can look in the

Image Size window for size confirmation) or Export at 72PPI. In either case, use the relevant optimization (available with both ways); it is also convenient to have 72PPI in the

Effect>Document Raster Effect Settings.

If you have pure vector artwork, you can relax a bit and have the artwork/Artboard at any size (the Artboard must have the same proportions as the final image), then use the Legacity Save

for Web and set either Width or Height in the Image Size and Apply (make sure the other value is also correct).

The Legacy Save for Web may be an old carthorse, but it knows its way home, even if the driver is drunk and sleeping it off in the hay in the back.