Bitmap images are based on pixels, which means that you will see pixels if it is enlarged more than 100%. You have all seen images that get jagged around the edges when enlarged.
Unlike bitmap vector formats can be enlarged indefinitely because they’re not based on pixels but on lines that can be stretched without loss of image quality.
When using a logo, a vector format will give you the flexibility you need to apply it to different size documents, including billboard advertising or large posters. With a vector format logo it is easy to change its color or the background color; whereas it is close to impossible to separate a logo from a background if it is a bitmap.
PDF and JPEG are both cross-platform compressed files, which means they will look the same on Mac or PC.
PDF (Portable document format) is best for compressing files that include text or vector images, while JPEGs are bitmap images mostly used for compressing photographs. Therefore, when compressing documents with text or vector images make sure you don’t save them as JPEG, as they will loose sharpness.
Low resolution images are really only for viewing on-line. For print images you will want high resolution, starting at 300 dpi. The biggest mistake people make with high and low res is that they think it will look good in print if it looks good on the screen. But if the image is in low res it will come out blurry and pixelated in print. You can’t increase the resolution of an image. That’s why it’s so important to have original photos in high res. If the file is under 1 MB, it is most likely low res.
RGB and CMYK are color schemes. RGB is what you see on the monitor; CMYK is what that translates to in print. RGB colors are more vibrant and vivid. They are not going to look the same in print. You shouldn’t be disappointed if you don’t see the exact same green on your business card that you saw on the monitor. Always ask for printed proof before committing to printing many copies.
The short answer here is DON’T USE THEM. Here’s why. First of all, most of these images are copyright protected; if you take an image you like and post it on your blog, for example, you can get fined for up to $4000. Secondly, if you decide to ignore this copyright law, most images have a watermark which is difficult to remove. And finally, these images are mostly low res and not good for printing.