Downloading and Saving Images
Databases - Saving mechanisms will vary, but there is usually a "save" icon. Some databases, like ARTstor, will download the file as a zipped folder, which you will need to unzip. Others download two files with the same name - oneis a jpeg and is the image, the other is an html file that contains the information about the image.
Web resources - For images on a website, simply right click on the image and choose "save as". Choose a file name that will enable you to easily find it again.
JPEG - This is the most common file type for images. Regular JPEGs use file compression, which means they are a smaller file size (i.e., they take up fewer MB on your hard drive). Large JPEGS are usually fine for webpages and projection, depending on the resolution and file size, but they may be inappropriate for printing.
TIFF - This is a higher quality, uncompressed file type. TIFF files tend to be larger, so if you just need simple copies of an image for presentations, it might be a good idea to convert them to JPEGs, as they will load faster.
Your image size will depend on the capabilities of the projector or monitor with which you will be displaying your images. For most classroom projectors, an image sized at around 1024x768 pixels will suffice. (This is the size of the entire frame thrown by the projector; if you are showing multiple images on one slide, they can be slightly smaller than this.)
There are many different tools for incorporating digital images into a presentation. Here are some of the most popular options: