You can use this browser-based tool to draw simple graphics which you can then paste into your IRC client, such as mIRC or WeeChat. The drawing options are still limited at the moment, and probably will undergo quite some changes still, but so far it's quite functional and its results have annoyed various IRC users already. ;-)
It's quite simple. First choose the desired canvas size you want to 'draw' on and click on the generate button. Next, click on any of the tiles of the canvas (the area with dashed rectangles). Clicking on a tile will pop up a color swatch from which you can pick one of the 16 default mIRC colours for that tile. Currently you can't pick fore- and background separately; the selected colour will be the fore- *and* background of the tile. The tile will also automatically be filled with a '█' character (U+2588 FULL BLOCK). In the future you'll probably be able to set the fore- and background colours separately, as well as pick the character you want to use, but for now this will have to suffice. Besides the canvas a preview of your drawing will also appear. In the future I might add multiple previews at various sizes, as well as on different coloured backgrounds.
As you 'draw', you'll notice the textarea at the bottom will automatically update. It will contain the IRC (colour) control codes as well as the block characters. By default the 'raw' format will be used, which means it doesn't contain any of the /say or /alias or /input insert commands. With the radio buttons you can switch between various IRC client formats you might want to use. For mIRC I've prefixed the lines with /say for more easy pasting into your alias files. The WeeChat format relies heavily on the multiline.pl script as WeeChat doesn't like pasting the control codes without using /input insert.
By default the tool uses spaces combined with reset colour codes for transparent tiles, but if you want, you can choose to have them replaced in the textarea with a fill colour of your choice using the Fill checkbox and the Fill Colour colour swatch above the lower textarea.