I have created a tikz picture as a standalone.
Can I export the tex file as an image instead of as a pdf?
I achieved the best results with
pdftoppm, the anti-aliasing is much better than imagemagicks
$ pdftoppm -png -r <dpi> document.pdf > document.png
I usually use 300 or 600 dpi.
pdftoppm comes with
poppler-utils on Ubuntu).
If you are using Texmaker, right-click on the pdf, then choose
convert page to png image choice. Note: if you intend to include this image in your tex file, then you can include the pdf directly. As a matter of fact, the quality of the pdf is better than converting the pdf to an image.