Possibly a feature requestBrowser Info Environment
Open up the view.php file, in your application/blocks/your_block_handle directory. You will see something like this:
<?php echo Page::getCurrentPage()->getAttribute('thumbnail'); ?>
Change it into this:
<?php echo Page::getCurrentPage()->getAttribute('thumbnail', 'display'); ?>
That will actually display the page attribute, instead of retrieving the raw value (array/object). The image attribute will only display a download link though, so not sure if you'd want that? If not, then you'd have to be doing something else (custom).
What is always the case though, if you select page attribute, is that you DO NOT have to select anything yourself in the block. It will take the attribute assigned to the page. So you will have to select a value for the page. If you simply want to select an image within this very block, select the "Image" field type please! That field got everything you need, resizing, cropping etc. etc.
As for the selecting this every time, that is incorrect. We are talking about a page attribute, and this will not be set using a block. You will have to set page attributes in the "Attributes" section of the page. A block is not meant for that. Block Designer will only let you make use of the "echo'ing" of a specific attribute's value. Usefull if you need the name of the page, the link or a file or whatnot.
If you want to select an image within the block, you can use the "Image" field type of Block Designer. This is a fully featured field type with cropping, resizing, linking etc. etc.
Let me know if you have any more questions!