I would like to print good images in A4 document (300 or 600 dpi). Using an external software,I have to merge differents big images to create a bigger one so I have to resize it with the same software (e.g. Gimp). I would like to know the best width (for example, for use them in a A4 report or a book class) so I can directly resize this big image to the proper size with the external software.
I wanted to post this as a comment, but I ran out of space.
There is always some confusion on the resolution of graphics. Usually the smaller the print gets the higher resolution is required. The
dpi actually stands for dots per inch an is a printer specific detail. Graphic programs usually use
ppi pixels per inch. It is often referred to as the same, but it is not.
Resizing in the source document is only a good option if you are using vector graphics, which will be directly rendered by the printer.
In all bitmap graphics you schould try to resize it with an actual graphic programm. Usually there is a connection between dots and pixels. Many common printers use 1dot(per colour) = 1 pixel, some can do more.
For an optimal result you just need to know how large your picture shall be in the document. I it is 15 inches width and your printer can do 300 dpi, and you say 1 dot = 1 px, then 4500 pixels is the desired width.
You can then include your graphic without resizing it. However, if you are not printing on a high quality printer, this is fairly unnecessary.
There's an interplay between graphics format and original graphic size and desired print size here which makes answering this question pretty much impossible.
As an initial starting point, my suggestion would be to always scale the graphics to
\textwidth when placing them --- then, if need be you can re-work the graphics (say it's vector and the line widths have become too anemic, so one should then increase the stroke weight, or it's a pixelmap and is grainy 'cause of the re-sizing, so one should source higher-resolution original, &c.)
It is left as an exercise for the reader to create a more generalized version which will handle portrait images and scale them to
Printing is not a very simple challenge. You have to look for several things, for example all your images should use the same color scheme, for example cymk. You should prepare all images to be included without resizing, because resizing usually results in a smaller image quality.
All of this depends on the printing system that is used to print your document. So the best way is to ask your printing company what you have to do. You can show them a current pdf version of your document and they can tell you what you should change. If you have Adobe Acrobat pro you can use the build-in tool preflight to check your pdf file. Otherwise the printing company will do it for you.