For the component palette you should provide images at the sizes 24x24, 36x36 and 48x48; they must be named like the component they are assigned to, and the medium and large images must have a name appendix "_150" and "_200" (for 150% and 200% size). Example: A component "tmybutton" requires icons named "tmybutton.png", "tmybutton_150.png" and "tmybutton_200.png". I am recommending the png format here because it has built-in and well-supported alpha-channel transparency.
You are asking about an application to draw these icons. Roland Hahn, the author of the icons recently used by Lazarus, recommends Inkscape. This is a vector-graphics application rather than a pixel-graphics application. But since images drawn by Inkcape can be exported as png in any size. Of course, you can also use a pixel-graphics application, such as
LazPaint or
GreenfishIconEditor.
lazres has nothing to do with image generation. What it does is to create a .res file out of the individual icons. It is delivered along with Lazarus, but you must compile it yourself; its Pascal sources are contained in the folder "tools" of the Lazarus installation. Have a look at the various batch files and shell scripts in folder "images" of the Lazarus installation to learn how to use it. Basically: "lazres buttonimages.res tmybutton.png tmybutton_150.png tmybutton_200.png" in order to create a res file "buttonimages.res" containing the png images following in the parameter list. Or "lazres buttonimages.res @filelist.txt" where "filelist.txt" is a text file with the filenames of the images to be included in each line, created by "dir /b *.png > filelist.txt".
BTW, when you use the extension .lrs in the resource file argument of lazres you will create an old lrs resource file. But since Lazarus nowadays supports the Delphi-compatible res files this is not recommended any more.
Read
https://wiki.lazarus.freepascal.org/How_To_Write_Lazarus_Component on how to create a Lazarus component.
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