What about resolution?
You don't need to worry about type. Headings and text will enlarge OK. To get technical for a second, these are vector images and enlargement is not an issue with them.
With pictures, logos and other graphics (called bitmap images), you need to capture or create them at a high resolution. How high? Well the rule is that the image resolution should be 300ppi at the final print size. But very often this is simply not feasible because the resulting file sizes become too large and unmanageable. So the 300ppi rule is broken every day! Especially by us!
If you're scanning your own images, set the resolution to 300ppi and scan the image as large as you can while still keeping the file size reasonably small. Less than 3MB is a reasonable size. If you're photographing your images, adjust the camera settings to give the best quality images possible. For other peoples' images, the best advice is to obtain the highest possible quality image you can from the originator. What you really need to avoid, if at all possible, is using graphic files taken directly from your computer screen. These are generally 72ppi resolution and they will not enlarge successfully.
And be careful when trying to improve your image files! Simply increasing the resolution of an existing low resolution file to 300ppi will only result in a bigger file, not necessarily a better file. If a picture looks pixellated on your proof, then it will almost certainly look worse on an enlargement.