Deep learning-based light field image super-resolution methods have witnessed remarkable success in recent years. However, most of them only focus on the encoder design and overlook the importance of upsampling process in decoder part. Inspired by the recent progress in single image domain with implicit neural representation, we elaborately propose spatial-epipolar implicit image function (SEIIF), which optimizes upsampling process to significantly improve performance and supports arbitrary-scale light filed image super-resolution. Specifically, SEIIF contains two complementary upsampling patterns. One is spatial implicit image function (SIIF) that exploits intra-view information in sub-aperture images. The other is epipolar implicit image function (EIIF) that mines inter-view information in epipolar plane images. By unifying the upsampling step of two branches, SEIIF extra introduces cross-branch feature interaction to fully fuse intra-view information and inter-view information. Besides, given that line structure in epipolar plane image integrates spatial-angular correlation of light field, we present an oriented line sampling strategy to exactly aggregate inter-view information. The experimental results demonstrate that our SEIIF can be effectively combined with most encoders and achieve outstanding performance on both fixed-scale and arbitrary-scale light field image super-resolution. Our code will be available upon acceptance.