Da documentação do WinSCP intitulada: Modos de transferência , vejo que há 2, modo binário e modo de texto para transferir um arquivo.
===== Server-side Text File Format =====
There are two options how to support text mode transfers. The first option is that the client (WinSCP) knows directly the text file format used by the server and converts the file to the format before transfer. The second option is that there is some in advance agreed canonical format to which the client converts the file before transfer and from which the server converts it (if necessary) after transfer to its own format.
The second option is more universal, but it is supported only by [[sftp|SFTP-4]] and newer and [[FTP]]((Actually with FTP, file is uploaded unmodified, anticipating the server to be able to convert it to its own format.)).
The first option is used by WinSCP for [[SCP]] and SFTP-3 and older protocols. It this case the client (WinSCP) must directly know and support the server-side format. Generally there are plenty of text file formats, almost every platform has it own format. WinSCP supports two of the most used formats, Unix and Windows. You can select between them on //[[ui_login_environment#end-of-line_characters|Environment page]]// of Advanced Site Settings dialog. As most SSH servers are run on Unix system, generally you may leave the default Unix format.
Então eu acredito que você tem o modo automático, que deve detectar o tipo png por padrão. Caso contrário, suspeito que o problema possa ser este artigo intitulado: Tópico "Arquivos de byte do WinSCP FTPing ZERO" ou este artigo intitulado: Por que meu arquivo enviado tem 0 bytes? .