ofFile Binary Mode

try using read instead of get. get is meant to be used to read strings and it'll stop after the first breakline: http://www.cplusplus.com/reference/istream/istream/get/

another possibility is to use ofBuffer to read the whole file to memory and then just iterate it in memory which will be faster even if it uses more memory but unless the file is too big it's probably worth it:

ofFile file(path_to_some_png, ofFile::ReadOnly, true);
ofBuffer buffer(file);
for (int i = 0; i < CHECKSUM_BLOCK_SIZE; i++) {
    cout << (unsigned)buffer.getData()[i] << endl;
}