Testing how upload to blog works on Opera Mini 5 beta 2, and using MyOpera's 'Add new photo' feature displayed on mobile phones.
And it seems to work just fine. The only problem is to find how to downsize the photo before posting. Otherwise I will use a lot of upload time for nothing. Uploaded images are downsized on server after upload finishes. That's why there's not much point to upload the full size image.
So far I haven't found a way to downsize the photos on my Sony Ericsson K800 phone. I guess it's just impossible. One option could be to post blog articles via MMS, because then SE K800 gives an option to downsize the photo before posting. However, using MMS to post images does not currently work at MyOpera.