Do graphics need to be 300 DPI?

Web images are always 72 DPI (monitors "see" images at around 72-100 something DPI). So it doesn't matter if a file is 72 or 300 DPI, they still would look the same.

Dpi or dot per inch is only relevant when you are actually printing, what determines the file size are the pixel dimensions. So, if you are keeping the same numbers of pixels, you are not actually changing a low res to a high res file. When you print, if you are telling the printer to print to a certain size of paper, it will set the dpi of the file to cover the size that is required.
What does this mean?
A digital image is measured in pixels. Now whether you set dpi to 72 or 300 the pixel dimensions remain the same e.g let's say for example an image is 1500 x 1700. At 72 dpi its 1500 x 1700.
If you change the dpi without resampling its still 1500 x 1700.
The image quality hasn't changed. The image is bit for bit identical.
By changing the dpi to 300 without resampling, you can now print it at its natural size without any reprocessing or quality loss at all.

Meaning what is important are graphic dimensions (pixelsize). Low resolution versus high resolution.This means if you take a 1500x1700 pixel image and resize it to 3000x3000 it will look pixeled, unless you use photo-enhance software as photozoom. But resample a 72 DPI image to 300 DPI will not change the quality of the image.
A low resolution image will be smaller in filesize (kilobytes) than a high resolution one.
Therefore the larger the files in pixel dimensions, the sharper they will be when resized to graphic design as scrapping. However 300 DPI versus 72 DPI have exactly the same filesize in kilobytes.
So unless you wish to print your creations, DPI has no meaning at all.

My PSP (and therefore my graphics) by the way is set on 300 PPI (pixels per inch).

Irfanview is a little powerful tool for resize/resample from 72 to 300 DPI and it is free!

Some articles explaining it better than I did :)

Why are my images only 72 dpi?

What is the difference between 72 and 300 dpi?

DPI is a form of measurement that defines the clarity of an image being printed. It is measured by the number of dots that fit into one square inch. The higher the dpi, the smaller the dots, the sharper the image. The lower the dpi, the bigger the dots, and the less clear the image. The standard for web is 72 dpi; however, the standard for print is 300 dpi. In other words, if you were to print a 72 dpi web image, it would come out looking really poor, yet on the web it would look really good.

This is due to the fact that paper is measured in inches. Therefore, the number of dots per inch highly affects the quality of an image when it is being printed. On the other hand, monitors measure in pixels which makes very little difference in the quality of a digital image.

by Pat Smith & Cynthia Mackey Connect 2 the Web

DPI Secrets Revealed