Images for documents
Do you want to add pictures to your report?
Most photographs you will put on a report/paper/book will be rather small in size. If you do not resize them, someone else will.
What size picture do I need ?
Unlike posters a picture for a document or book will not will not fill your entire page. A typical A4 page (similar to US Letter) is about 30 cm (11.75 inches) high and 21 cm (8.25 inches) wide. You usually have some margins on all sides, and used in standard (portrait) orientation, the more common way is to have a picture that fills half the printable area with some margins. This is about 15cm x 10cm (or 6 inches by 4 inches) or even a bit smaller. It is not a coincidence that this is the standard photo print size.
What you technically need is at most 300 DPI, which translates to a picture about 2 Megapixels for a proper half page picture in a document and twice as that for a picture that is printed sideways to fill most of the printable area. Basically any picture on this wiki (which are resized to 1280 pixels wide) would work well (translates to a bit more than 200 DPI), and anything above 2000 pixels wide would be an overkill.
While printers often claim fantastic numbers (600-1200 DPI or more) there is a difference between the dots on your image (which have different color components) and the dots on paper where color is not really mixed, but printed in a pattern with dots to create the color. Unless you are using a specialist print having more resolution on your picture will not really help.
If your resolution is higher, either the software you use for your document, your printer driver or the printer itself will resize your image to fit the capabilities of your printer.
An example
The following is from the Europractice activity report from 2019 printed on high quality glossy paper.
On this page there are three pictures, two of which are photographs. This is a regular looking picture that you would find in any document, notice that it is much smaller than the half page photos we were talking about before. If you measure the size of the photo you will realize that it measures about 3 inches wide.
Let us zoom close to the picture of the print, the ruler at the bottom shows millimeters
And if we zoom even closer you can see the individual dots from the print. Notice the colors are yellow, cyan, and magenta, not red, green, and blue. The ruler is again in millimeters. What you see is a bit less than 0.2 inches (5.12mm), you can discuss about the DPI but it is effectively around 200 DPI.
This is the actual image I used for the print. The original is a 12 Megapixel image shot by a Nikon D700.
And finally this is the 1:1 crop from the image around the same part that I have above (1280 x 850 pixels). You will notice that I have more than enough resolution to be printed without any issues.
Basically 1 Megapixel for a 3inch image, and you will not notice anything even if you look closely.
These pages are for Amateur Photographers and not really for seasoned photographers and professionals. I have no affiliation or commercial interest with any brand/make. I write from my own experience. I ended up using mainly Nikon, so I am more familiar with this brand than others. See price for notes on pricing as well as photography related links.