EDITO: I'm creating a website and the images they gave me are too big. In Windows I used Photoshop to reduce the size, or maybe it is better to say scale the size, because I want to keep the whole image, in a smaller size, to be able to put them in the grid.
I ask for advice here, because I never use an editor in Ubuntu, I see all that there is in: Ubuntu Software, but not what would be the most appropriate for professional image editing, in the creation of a website, and to be able to give the best possible appearance to the images. In this case, reduce the size of some images, or perhaps it would be better to say, re-dimension. Thank you.
GUI Workaround:
Gimp (GNU Image manipulating program) is equivalent to Photoshop in Gnu/Linux distributions.
Open your image in Gimp then from "File" menu select "Export as", choose "JPEG" as export format and click on Export
It will results this window:
Adjust your desired values and save the result.
You'll get similar settings for other formats if any is available, for example png:
In this way you can work around your images similar to Photoshop.
CLI
You can also achieve this goal using command line using "imagemagick" package.
It's all based on your needs and preference, but a common image viewer with basic editing tools is "gwenview" and for Adobe Photoshop replacement you can use "GIMP".
In Gwenview, Edit>Resize
Also take a look at this topic to see how to batch resize images using imagemagick: How can I scale all images in a folder to the same width?