"online log" is a Web application. It is installed on the Web server of a Web hosting provider.

Necessary is Web space with PHP starting from version 4.3 (Hypertext Preprocessor for dynamic Web content) and MySQL starting from version 3.23 (data base binding).
See also
PHP Web space / host search.

Yes, this concerns at the clearest embedded file upload for pictures and the ADIF data import. (Amateur Data Interchange Format)
1. File upload must be generally permitted.
2. The maximally transferable file size specifies the Provider with the configuration value "upload_max_filesize".
One should inquire this value before conclusion of a contract.
For example: an ADI file for the ADIF import with 50.000 records can be estimated with a file size of approximately 10 MByte.
If the files are too large for the internal upload, an external FTP program must be used. (FTP: file transfer protocol)
The current PHP configuration of the own Web space can be looked up in the Web administration of
"online log", menu item "PHP Info".

Yes, you can register your own headline image and event standard image in the Web administration.
The dimensions of pictures are given. Headline image: width x height = 738 x 60 pixels, event standard image: width x height = 440 x 330 pixels.
The file formats JPG, GIF and PNG can be used. The images are edited with an image-editing program like "PhotoFiltre" and transferred to the Web server with the internal image upload.

Yes, the colors can be varied in the style template "User defined style".
The change takes place with the help of a text editor in the file "style_z_userdefined.php", which is to be found in the directory "layout".
The phase of editing takes place at the local computer. Subsequently, the modified file is copied back to the server with a ftp program.
Now go to the Web administration, menu item "Settings" and select "User defined style".